Before being able to boot from another partition (another operating system), you have to select it in some way.

The are actually no standardized techniques for this. Without a bootmanager, you have to "activate" this partition in the partition table before booting another operating system.

Of course, this is quite awkward. That is the reason so many so-called "bootmanagers" are now available on the market. These programs allow users to select a partition and with that an operating system during booting.

The hard disk partitioner PartitionStar may be used to switch the active partition via command line options.